(Nanowerk Spotlight) Diatoms are a major group of hard-shelled algae and one of the most common types of phytoplankton. A characteristic feature of diatom cells is that they are encased within a unique cell wall made of silica. Silicate materials are very important in nature and they are closely related to the evolution of living organisms. Diatom walls show a wide diversity in form, some quite beautiful and ornate, but usually consist of two symmetrical sides with a split between them, hence the group name. There is great potential for the use of diatoms in nanotechnology. This potential lies in the pores and channels which give rise to a greatly increased surface area, and the silica structure which lends itself to chemical modification. In addition there is a huge variety in the sizes and shapes of diatoms available, providing scope for the selection of a particular species of diatom tailored to a particular requirement. Researchers in the UK have demonstrated that the silica walls of diatoms can be used for the attachment of active biomolecules, such as antibodies, using either primary amine groups or the carbohydrate moiety. These modified structures can, therefore, be used for antibody arrays or for use in techniques such as immunoprecipitation.

"Nature provides us with myriad solutions if we can only find the right questions to ask" Dr. Helen Townley tells Nanowerk. "Bionanotechnology is demanding systems which are cheaper, smaller, cleaner, and faster. A major goal for bionanotechnology is the development of microscale total analysis systems (µTAS) or lab-on-a-chip. An ideal solution is to exploit the cheap nanostructures which are provided by Nature. Unlike most manufacturing processes, diatoms achieve a high degree of complexity and hierarchical structure under mild physiological conditions. Diatoms also show remarkable mechanical strength which is useful for industrial applications. In addition they are easy to culture, have a fast reproductive rate, a dominant vegetative state of reproduction, low culture costs and present a virtually unlimited renewable source."

For their experiments, the Oxford scientists used Coscinodiscus wailesii, a large centric diatom that has a regular pattern of pores. In the images below, the silica surface can be seen to be perforated with pores averaging 1 µm in diameter (1B). These pores lead to a channel through the silica of the valve ending in a base plate with further perforation. The base plate pores are around 200 nm in diameter (Fig. 1C). Townley explains that this channel and base plate structure results in a high surface area for any given valve diameter, while allowing free passage of biological molecules (bovine serum albumin, for example, is approximately 7 nm in diameter).

SEM images of Coscinodiscus wailesii showing A) the two valves, B) pores (foramen) in the frustule surface, and C) detail of the sieve plate. (Reprinted with permission from Wiley-VCH Verlag)

What Townley and her collaborators, Dr Andrew R. Parker and Dr. Helen White-Cooper, now demonstrated is that the diatom frustule can be chemically modified for the attachment of antibodies, and that the attached antibodies retain biological activity.

What is also interesting from a fabrication point of view is that these silica structures are produced in diatoms without the need for industrial chemical processes, using only light and minimal nutrients and, therefore, generate an exceptionally cheap and renewable material.

Since attachment of bioactive molecules to a solid support is key to lab-on-a-chip technologies, Townley and her colleagues started by investigating methods for covalently tethering antibodies to the diatom silica matrix and assaying their biological activity when attached.

"We demonstrated the successful attachment of an anti-IgY (IgY: immunoglobulin Y) antibody raised in rabbit to the silica surface of the diatom using silane chemistry" explains Townley. "While we have shown that it is possible to tether the antibody to the diatom surface via the antibody amino groups, conjugation via these groups gives the potential for multiple attachments with resulting conformational distortions. We therefore investigated an alternative tethering approach – to attach via the carbohydrate rather than protein components of antibody molecules." Using this method the scientists were able to bind mouse anti-tubulin to the diatom surface.

Townley notes that an ideal µTAS would be able to detect multiple antibodies or antigens present in a complex mixture such as sera. "As a proof of principle, we tested whether the diatom surface could be used to tether two different antibodies" she says. "In this experiment we tethered a mix of normal rabbit serum and purified Ig-Y, and detected the attached antibodies with FITC-conjugated
(FITC: fluorescein isothiocyanate) anti Ig-Y and/or Cy5-conjugated anti-rabbit by using confocal microscopy. We showed that two antibodies can be successfully attached, and detected, on the surface of a single diatom."

"Logically, our approach can be extended to multiple antibodies from a serum sample for example" says Townley. "This could then be probed with multiple secondary antibodies conjugated with different fluorescent dyes."

The next step for the scientists is to incorporate these modified nanostructures into a workable design which could provide a cheap and disposable analysis system.

Fuhrmann et al. showed that the silica cell wall of the diatom Coscinodiscus granii can be regarded as a photonic crystal slab waveguide ("Diatoms as living photonic crystals"). Furthermore, they present models to show that light may be coupled into the waveguide and give photonic resonances in the visible spectral range.

Although this has yet to be demonstrated experimentally, Townley says that, in addition to the applications that could result from this in terms of physics and engineering, it is also fascinating in terms of biology. For instance, does the organism in some way utilize the properties of the frustule to enhance its photosynthetic capacity?

If you are interested in this topic, Townley and Parker reviewed the biomimetics of photonic nanostructures in a recent progress article in Nature Nanotechnology ("Biomimetics of photonic nanostructures").